1. Field of the Invention
The present invention relates to logical comb filter mainly used for a Y/C separation in video instruments such as a video tape recorder/reproducer, a television receiver, etc., and more specifically to a logical comb filter inexpensively constructed for processing a video signal in a digital manner. Furthermore, the present invention relates to a chroma signal separation signal constituted of such a logical comb filter, for separating a chroma signal from a NTSC composite video signal.
2. Description of related art
A logical comb filter for separating a chroma signal from a NTSC composite video signal has been basically composed of maximum value circuits and minimum value circuits. In the case that this filter is formed of an analog circuit, each of the maximum value circuits and the minimum value circuits can be formed of a simple circuit constituted of two transistors. However, In the case that this filter is formed of a digital circuit, the digital circuit does not generally have either the maximum value circuit or the minimum value circuit. Therefore, each of the maximum value circuits and the minimum value circuits is formed of a combination of NAND circuits and NOR circuits, or a combination of comparators and selectors. Ordinarily, comparators and selectors are used for constitute the maximum value circuits and the minimum value circuits.
Therefore, in the case that the logical comb filter is formed of an analog circuit, the logical comb filter can be constituted of a relatively small number of transistors, but in the case that the logical comb filter is formed of a digital circuit, the logical comb filter needs a considerable number of transistors. For example, assuming that the signal bit length is 8 bits in accordance with general digital processings, the comparators need 60 gates, and the selectors need 22 gates, and the whole of the filter requires 432 gates including addition circuits. In this connection, a CMOS gate needs four transistors for one gate.
Recently, the logical comb filters have been incorporated in domestic video tape recorder/reproducers. In view of mass productivity (no adjustment) and stability to change in time, inclination of forming the logical comb filter in the digital circuit is increasing. However, if the logical comb filter is formed in the digital circuit, the circuit size becomes large and expensive as mentioned above.
On the other hand, a chroma signal separation circuit includes a type in which the characteristics of the separation filter is changed on the basis of a large/small relation of chroma signal difference between a current line and a preceding line and a succeeding line.
For example, an input composite video signal is supplied to and delayed by a first one-line delay circuit, and then, an output of the first one-line delay circuit is supplied to and delayed by a second one-line delay circuit. In addition, the input composite video signal, the output of the first one-line delay circuit and an output of the second one-line delay circuit are inputted to first, second and third bandpass filters, respectively. Respective outputs of the first, second and third bandpass filters are called a "succeeding signal", a "main signal" and a "preceding signal", respectively.
The main signal and the succeeding signals are added to give a succeeding comb signal, and the main signal and the preceding signals are added to give a preceding comb signal. A mean value of the succeeding signal and the preceding signal gives a three-line comb signal. On the other hand, a difference between the main signal and the preceding signal gives a preceding difference signal, and a difference between the main signal and the succeeding signal gives a succeeding difference signal. One signal is selected from the preceding comb signal, the succeeding comb signal, and the three-line comb signal, on the basis of a selection signal which is obtained by multiplying the preceding difference signal and the succeeding difference signal by coefficients and by processing the result of the multiplication.
One conventional chroma signal separation circuit has been configured to select one of the preceding comb signal and the succeeding comb signal corresponding to a smaller one of the preceding difference signal and the succeeding difference signal, and to select the three-line comb signal if the preceding difference signal and the succeeding difference signal are substantially the same.
This conventional chroma signal separation circuit has been disadvantageous in which if the above mentioned coefficients are near to 1 (one), when a pattern of a three-line signal change of the preceding line, the current line and the succeeding line is a step form (in which the signal becomes larger or smaller in the order of the preceding line, the current line and the succeeding line), chattering occurs in switching of the comb signal. As a result, a picture of a finally displayed image becomes unnatural. Specifically, nonuniformity occurs in cross-color and dot obstruction, and therefore, a picture deterioration becomes remarkable in comparison with the case of no nonuniformity. On the other hand, if the coefficients are made sufficiently larger than 1 (one), when the signal change pattern is a pulse form (in which the main signal is larger or smaller than both of the preceding signal and the succeeding signal), a level variation occurs.
Another conventional chroma signal separation circuit has been so configured that the NTSC composite video signal is supplied to the bandpass filter and thereafter is supplied the above mentioned logical comb filter so as to obtain a chroma signal. In this conventional chroma signal separation circuit, when the three-line signal change pattern is the step form, no chattering occurs in the switching of the comb signals. However, the dot obstruction becomes to easily occur.